Breast cancer cells display variable expressions of hormone receptors (HRs), mainly estrogen receptors (ERs), progesterone receptors (PRs), and growth factor proteins such as human epidermal growth factor receptor-2 (HER2), and the malignancy encompasses the hormone receptor-based subtypes luminal A (Lum A), luminal B (Lum B), HER2-positive (HER2+), and triple-negative breast cancer (TNBC), each with distinct molecular characteristics and clinical behaviors [3,4,5]. The gene discussed is ESR1; the disease is breast cancer.